: Computer spies have broken into the Pentagon's £200bn Joint Strike Fighter project and copied information about its design and electronics systems.
F-35 Lightning II fighter
The Wall Street Journal (WSJ) said intruders were able to copy and siphon off data potentially making it easier to defend against the craft.
The breach follows similar hacking attempts on the US air traffic control system.
Former US officials say the attacks appear to have originated in China although the exact identity of the hacker is unknown.
The scope of the damage to the US defence programme, either in financial or security terms, also remains unclear.
While the spies were able to download sizeable amounts of data related to the jet fighter, they were not able to access the most sensitive material.
This is stored on computers not connected to the internet.
Women's Hands Typing On Keyboard
A global battle over data networks
The Joint Strike Fighter, also known as the F-35 Lightning II, is the costliest and most technically challenging weapons programme the Pentagon has ever attempted.
Britain announced it would purchase three of the aircraft for operational testing earlier this year.
The plane, led by Lockheed Martin Corp, relies on 7.5 million lines of computer code.
The Government Accountability Office says this is more than triple the amount used in the current top Air Force fighter.
Six current and former officials familiar with the matter confirmed that the fighter programme had been repeatedly broken into.
And the Air Force has launched an investigation.
The latest intrusions provide new evidence that a battle is heating up between the US and potential adversaries over data networks.
A recent WSJ report said computers used to control the US electrical-distribution system, as well as other infrastructure, have also been infiltrated by spies abroad.
Attacks like these - or US awareness of them - appear to have escalated in the past six months, said one former official briefed on the matter.
"There's never been anything like it," the source said, adding that other military and civilian agencies as well as private companies are being affected.
"It's everything that keeps this country going."
:: For more on this story click here to go to the Wall Street Journal